Ignition circuit for spark plugs of internal-combustion engine

ABSTRACT

The ignition circuit of a spark plug of an internal-combustion engine includes a storage condenser in series with a controlled rectifier which is periodically triggered under the control of an engine-driven cam switch by a blocking oscillator comprising a transistor with grounded emitter in series with the primary of a coupling transformer. One secondary winding of that transformer is inserted, together with a pair of diodes, in a charging path for the storage condenser while another secondary winding lies between the emitter of the transistor and its base which is connected through a feedback resistor to the collector thereof and via an R/C network to the gate of the controlled rectifier. During the open phase of the cam-switch cycle, the base is grounded through the last-mentioned secondary winding which is so poled as to intensify a firing pulse capacitively transmitted to the base upon the opening of the switch; at the same time the controlled retifier is turned on to discharge the previously charged storage condenser through the ignition circuit, whereupon the transistor cuts off and the condenser is recharged by the reversed transformer voltage.

United States Patent [191 Del Zotto 1March 20, 1973 [75] Inventor:Giorgi0DelZ0tto,Milan,ltaly [73] Assignee: Ates Componenti ElettroniciS.p.A.,

Milan, ltaly 22 Filed: March 15, 1971 21 Appl.No.: 124,173

[30] Foreign Application Priority Data Primary Examiner-Herman KarlSaalbach Attorney-Karl F. Ross The ignition circuit of a spark plug ofan internal-combustion engine includes a storage condenser in serieswith a controlled rectifier which is periodically triggered under thecontrol of an engine-driven cam switch by a blocking oscillatorcomprising a transistor with grounded emitter in series with the primaryof a coupling transformer. One secondary winding of that transformer isinserted, together with a pair of diodes, in a charging path for thestorage condenser while another secondary winding lies between theemitter of the transistor and its base which is connected through afeedback resistor to the collector thereof and via an R/C network to thegate of the controlled rectifier. During the open phase of thecam-switch cycle, the base is grounded through the last-mentionedsecondary winding which is so poled as to intensify a firing pulsecapacitively transmitted to the base upon the opening of the switch; atthe same time the controlled retifier is turned on to discharge thepreviously charged storage condenser through the ignition circuit,whereupon the transistor cuts off and the condenser is recharged by thereversed transformer voltage.

10 Claims, 1 Drawing Figure PATENTEUHARZOIHB 3 721,224

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Giorgio Del Zoffo INVENTOR Attorney IGNITION CIRCUIT FOR SPARK PLUGS FINTERNAL-COMBUSTION ENGINE My present invention relates to a system forintermittently firing a spark-discharge device, such as a spark plug inan internal-combustion engine of an automotive vehicle.

The ignition circuit of an automotive spark plug generally includes astorage condenser which is alternately charged from a direct-currentsource, such as the vehicular battery, and discharged through a step-uptransformer generating the high voltage necessary to break down thespark gap. The timing of the discharge is controlled by a switchoperated by the cam shaft of the engine, which in a four-cylinder modelmay execute about 7,000 revolutions per minute.

It has already been proposed to charge the storage condenser through acoupling transformer in the output circuit of a monostable blockingoscillator periodically tripped by the cam-operated switch whereby thecontacts of the latter are traversed by only a small fraction of thecharging current and are thus less subjected to wear. In such a system acontactless switching device, such as a controlled rectifier of thesilicon type (SCR), may be used to close the discharge circuit of thestorage condenser at a predetermined point in the cycle. Prior systemsof this kind, however, were relatively cumbersome and expensive.

It is, therefore, the general object of my present invention to providea simplified ignition system of this character for the firing ofautomotive spark plugs and related devices.

This object is realized, pursuant to the present invention, by theprovision of a three-winding coupling transformer having its primarywinding in series with the output electrode (collector) of a transistorforming part of the blocking oscillator; one secondary winding of thetransformer is connected between the input electrode (base) and thecommon electrode (emitter) of the transistor with such a polarity as tointensify both blocking and unblocking potentials applied to that inputelectrode via a resistive feedback connection, another secondary windingof the same transformer lying in the charging circuit of the storagecondenser whose discharge circuit includes a normally open electronicswitch (such as a silicon controlled rectifier) with a control circuitextending from the transistor input to close that switch in the presenceof an unblocking potential, i.e. when the transistor conducts. Thus, areversal of the cam-operated switch to bias the transistor intoconduction causes a rising current flow through the coupling transformerwhich regeneratively energizes the transistor input (its base) and isalso communicated to the control electrode for the electronic switch(i.e. the SCR gate) to discharge the previously charged storagecondenser; as the transistor and/or the core of the couplingtransformers saturates, the regenerative feedback diminishes and thetransistor begins to cut off, the resulting polarity reversal in theseveral transformer windings resulting in the flow of charging currentinto the condenser whose discharge circuit is broken at this point bythe reopening of the electronic switch.

The invention will be described in detail hereinafter with reference tothe accompanying drawing the sole FIGURE of which diagrammaticallyillustrates a representative embodiment.

The drawing shows an ignition system for a spark plug 10 of anautomotive engine, not further illustrated,

which drives a shaft 11 with a cam 12 periodically opening normallyclosed switch P. A source of direct current 13, such as the battery of avehicle powered by the engine, generates a positive voltage Vb on a busbar 14 and has its negative terminal grounded. Bus bar 14 is connected,within a blocking oscillator generally designated Ob, to the collectorof an NPN transformer 01 through the primary winding N, ofa couplingtransformer Tr having a core 15 with two further windings thereon, i.e.,a feedback winding Np and a load winding N The relative polarities ofthe currents flowing through these windings have been conventionallyindicated by dots.

Switch P lies between ground and bus bar 14 in series with a leakageresistor R1. The junction of this resistor with switch P is connected tothe base of transistor Q1 through a circuit including, in series, acapacitor C1 and three resistors R2, R3 and R4. Two diodes DI and D2 areshunted across resistors R2 and R3, respectively, with a polarityallowing positive pulses from capacitor C1 to bypass the resistors R2and R3 in flowing to the base of the transistor 01 to unblock it. Thisbase is normally maintained at emitter potential (ground) throughtransformer secondary N F inserted between ground and the junction ofresistors R2 and R3, the same junction being connected to the collectorof the transistor by way of a feedback resiStor R6. An integratingcircuit, consisting of a resistor R5 in parallel with a condenser C3,lies between the base and the emitter of transistor Q1 to shunt voltagetransients to ground.

The other transformer secondary N, is connected in a charging circuitfor a storage condenser C4 in series with a pair of diodes D3 and D5. Adischarge circuit for condenser C4 includes the primary of avoltage-step-up transformer THT whose secondary feeds the spark plug 10its primary lying in series with an SCR D4 having its gate connected tothe junction of resistors R3 and R4 in the input circuit of transistorQ1 by way of an R/C network consisting of a condenser C2 and a resistorR7 in parallel therewith.

The operation of the system shown in the drawing will now be described.

As the rotating cam 12 opens the main switch P at a predetermined pointin its cycle, the previously grounded terminal of input capacitor C1 isdriven sharply positive by way of resistor R1 and transmits a pulse ofthe same polarity via diodes D1, D2 and the relatively small inputresistor R4 to the base of transistor Q1. The transistor thereuponstarts conducting so that the trickle current previously flowing throughits primary winding N in series with resistor R6 and winding Nprogressively rises to drive the transistor into saturation. The e.m.f.developed at the same time across secondary winding N is of suchpolarity as to maintain the base potential positive, this positivepotential being also communicated via circuit C2, R7 to the gate ofcontrolled rectifier B4 whereby the previOusly charged condenser C4discharges through transformer THT and fires the spark plug 10.

With saturation of transistor Q1 and/or transformer core 15, the buckingvoltage in winding N F decreases to apply a negative-going potential tothe base of transistor Q1. This reduces the current flow through primaryNp and rapidly builds up a negative blocking potential in the transistorinput until conduction ceases. At the same time the reversal of thepolarity of the voltage developed across secondary winding N energizesthe charging circuit for condenser C4 by way of diodes D3 and D5. WithSCR D4 cut off by the negative potential on its gate electrode, thesecondary current generated in winding N is prevented from circulatingthrough the controlled rectifier. Meanwhile, the switch P has beenreclosed by cam 12 so that the junction of resistor R1 and capacitor C1iS again grounded and the oscillator Ob constituted by transistor Q1 andassociated elements is inhibited until the switch P reopens in the nextcycle.

The provision of diode D5 enables the dissipation of residual energyelectromagnetically stored in the primary of transformer THT after theelectronic switch D4 has opened. On the other hand, the inclusion ofresistor R7 in the control circuit of SCR D4 insures the presence ofpositive gate voltage throughout the discharge phase, thereby preventinga premature cutoff of the controlled rectifier which could otherwiseoccur, for example, prior to the firing of the spark plug 10 as a resultof the counter-emf. developed across the primary oF transformer Tl-lT.This insures a complete discharge of the condenser C4 and preventsovercharges in subsequent cycles.

Resistor R3 and diode D2 protect the transistor Q1 and the controlledrectifier D4 from excessive negative voltages during switchover betweensaturation and cutoff.

The energy stored in the core of transformer Tr equals kLl where L isthe inductance of primary winding N and 1,, is the peak current flowingthrough that winding during conduction of the transistor. This energy, alarge part of which is transferred to the storage condenser C4, mayrange in magnitude between 50 and 75 mjoules with battery voltagesranging between 8 and 16 volts. With the same range of battery voltagesVb, the charging potential of storage condenser C4 may vary between 320and 390 volts. The output voltage developed across the secondary windingof step-up transformer THT may reach 24 KV if the resistance of thespark gap of spark plug 10 is on the order of 1 MO. With a 4-cycleengine operating at 7,000 RPM, the spark plug of each cylinder firesroughly at the rate of 60 cycles per second; the duration of the sparkdischarge is on the order of 200 [.L see, with a build-up period ofabout pace for the breakdown voltage across the spark-plug electrodes.These parameters are comparable to those realized with considerably morecomplex conventional ignition circuits of the same general type.

I claim:

1. A system for intermittently igniting a sparkdischarge device,comprising:

a source of direct current;

a blocking oscillator connected across said source, said oscillatorincluding a transistor with an emitter, a collector and a base, acoupling transformer having a primary winding inserted between saidcollector and a first terminal of said source,

said emitter being tied to a second terminal of said source, a capacitorin series with said base, switch means for al ernately applying ablocking potential and an unblocking potential from said source to saidcapacitor, and a resistive connection between said base and the junctionof said primary winding with said collector, said coupling transformerhaving a first secondary winding connected between said base and saidemitter for intensifying said unblocking and blocking potentials uponincipient conduction and cutoff of said transistor;

an ignition circuit for said sparledischarge device including acondenser and an electronic switch in series;

a charging circuit including another secondary winding of said couplingtransformer and diode means in series with said condensor; and

a control circuit for said electronic switch connected to said base forclosing said electronic switch in the presence of said unblockingpotential, thereby discharging said condenser through said ignitioncircuit, said other secondary winding being poled to recharge saidcondenser upon incipient cutoff of said transistor.

2. A system as defined in claim 1 wherein said electronic switch is acontrolled rectifier with a gate electrode connected to said controlcircuit.

3. A system as defined in claim 1, further comprising a resistivevoltage divider inserted between said capacitor and said base, saidresistive connection being joined to said voltage divider at a first tapthereof relatively remote from said base, said control circuit beingconnected to said voltage divider at a second tap thereof relativelyclose to said base.

4. A system as defined in claim 3 wherein said control circuit includesa capacitance and a resistance in parallel.

5. A system as defined in claim 3 wherein said first secondary windingis connected between said emitter and said first tap.

6. A system as defined in claim 5, further comprising a first diodeshunting said voltage divider between said capacitor and said first tap,and a second diode shunting said voltage divider between said first andsecond taps, said diodes being poled to pass said unblocking potential.

7. A system as defined in claim 1 wherein said transistor is providedwith an integrating network connected between said base and saidemitter.

8. A system as defined in claim 1, further comprising a leakage resistorconnected between said capacitor and said first terminal, said switchmeans being operable to connect said second terminal to the junction ofsaid leakage resistor with said capacitor for applying said blockingpotential to the latter.

9. A system as defined in claim 1 wherein said ignition circuit includesa step-up transformer in series with said condenser and said electronicswitch.

10. A system as defined in claim 1 wherein said device is a spark plugof an internal-combustion engine, said switch means comprising a camdriven by said engine.

i k l 4

1. A system for intermittently igniting a spark-discharge device, comprising: a source of direct current; a blocking oscillator connected across said source, said oscillator including a transistor with an emitter, a collector and a base, a coupling transformer having a primary winding inserted between said collector and a first terminal of said source, said emitter being tied to a second terminal of said source, a capacitor in series with said base, switch means For alternately applying a blocking potential and an unblocking potential from said source to said capacitor, and a resistive connection between said base and the junction of said primary winding with said collector, said coupling transformer having a first secondary winding connected between said base and said emitter for intensifying said unblocking and blocking potentials upon incipient conduction and cutoff of said transistor; an ignition circuit for said spark-discharge device including a condenser and an electronic switch in series; a charging circuit including another secondary winding of said coupling transformer and diode means in series with said condensor; and a control circuit for said electronic switch connected to said base for closing said electronic switch in the presence of said unblocking potential, thereby discharging said condenser through said ignition circuit, said other secondary winding being poled to recharge said condenser upon incipient cutoff of said transistor.
 2. A system as defined in claim 1 wherein said electronic switch is a controlled rectifier with a gate electrode connected to said control circuit.
 3. A system as defined in claim 1, further comprising a resistive voltage divider inserted between said capacitor and said base, said resistive connection being joined to said voltage divider at a first tap thereof relatively remote from said base, said control circuit being connected to said voltage divider at a second tap thereof relatively close to said base.
 4. A system as defined in claim 3 wherein said control circuit includes a capacitance and a resistance in parallel.
 5. A system as defined in claim 3 wherein said first secondary winding is connected between said emitter and said first tap.
 6. A system as defined in claim 5, further comprising a first diode shunting said voltage divider between said capacitor and said first tap, and a second diode shunting said voltage divider between said first and second taps, said diodes being poled to pass said unblocking potential.
 7. A system as defined in claim 1 wherein said transistor is provided with an integrating network connected between said base and said emitter.
 8. A system as defined in claim 1, further comprising a leakage resistor connected between said capacitor and said first terminal, said switch means being operable to connect said second terminal to the junction of said leakage resistor with said capacitor for applying said blocking potential to the latter.
 9. A system as defined in claim 1 wherein said ignition circuit includes a step-up transformer in series with said condenser and said electronic switch.
 10. A system as defined in claim 1 wherein said device is a spark plug of an internal-combustion engine, said switch means comprising a cam driven by said engine. 